arXiv subjects

Oleg V. Konovalov

Publications and source records attributed to Oleg V. Konovalov.

4 recordsLinked to original sources

arXiv2024

In Situ Characterisation of Graphene Growth on Liquid Copper-Gallium Alloys: Paving the Path for Cost-Effective Synthesis

Liquid metal catalysts (LMCats), primarily molten copper, have demonstrated their efficiency in the chemical vapour deposition (CVD) approach for synthesising high-quality, large-area graphene. However, their high melting temperatures limit broader applications. Reducing the temperature of graphene production on LMCats would lead to a more efficient and cost-effective process. Here, we investigated the effects of alloying copper with a low-melting temperature metal on graphene growth in real-time. We examined a set of liquid copper-gallium alloy systems using two complementary in situ techniques: radiation-mode optical microscopy and synchrotron X-ray reflectivity (XRR). Microscopy observations revealed reduced catalytic activity and graphene quality degradation in compositions with gallium domination. The XRR confirmed the formation of single-layer graphene on alloys with up to 60 wt% of gallium. Additionally, we detected a systematic increase in adsorption height on the alloys' surface, suggesting a weaker graphene adhesion on gallium. These findings propose a trade-off between layer quality and production cost reduction is feasible. Our results offer insights into the CVD synthesis of graphene on bimetallic liquid surfaces and underscore the potential of gallium-copper alloys for enabling the direct transfer of graphene from a liquid substrate, thereby addressing the limitations imposed by high melting temperatures of conventional LMCats.

physics.app-ph↗

arXiv2024

Operando Characterization and Molecular Simulations Reveal the Growth Kinetics of Graphene on Liquid Copper during Chemical Vapor Deposition

In recent years, liquid metal catalysts have emerged as a compelling choice for the controllable, large-scale, and high-quality synthesis of two-dimensional materials. At present, there is little mechanistic understanding of the intricate catalytic process, though, of its governing factors or what renders it superior to growth at the corresponding solid catalysts. Here, we report on a combined experimental and computational study of the kinetics of graphene growth during chemical vapor deposition on a liquid copper catalyst. By monitoring the growing graphene flakes in real time using in situ radiation-mode optical microscopy, we explore the growth morphology and kinetics over a wide range of CH4-to-H2 pressure ratios and deposition temperatures. Constant growth rates of the flakes' radius indicate a growth mode limited by precursor attachment, whereas methane-flux-dependent flake shapes point to limited precursor availability. Large-scale free energy simulations enabled by an efficient machine-learning moment tensor potential trained to density-functional theory data provide quantitative barriers for key atomic-scale growth processes. The wealth of experimental and theoretical data can be consistently combined into a microkinetic model that reveals mixed growth kinetics that, in contrast to the situation at solid Cu, is partly controlled by precursor attachment alongside precursor availability. Key mechanistic aspects that directly point toward the improved graphene quality are a largely suppressed carbon dimer attachment due to the facile incorporation of this precursor species into the liquid surface and a low-barrier ring-opening process that self-heals 5-membered rings resulting from remaining dimer attachments.

cond-mat.mtrl-sci↗

arXiv2022

X-ray scattering study of GaN nanowires grown on Ti/Al$_{2}$O$_{3}$ by molecular beam epitaxy

GaN nanowires (NWs) grown by molecular beam epitaxy on Ti films sputtered on Al$_{2}$O$_{3}$ are studied by X-ray diffraction (XRD) and grazing incidence small-angle X-ray scattering (GISAXS). XRD, performed both in symmetric Bragg reflection and at grazing incidence, reveals Ti, Ti$_{3}$O, Ti$_{3}$Al, and TiO$_x$N$_y$ crystallites with in-plane and out-of-plane lattice parameters intermediate between those of Al$_{2}$O$_{3}$ and GaN. These topotaxial crystallites in Ti film, formed due to interfacial reactions and N exposure, possess fairly little misorientation with respect to Al$_{2}$O$_{3}$. As a result, GaN NWs grow on the top TiN layer possessing a high degree of epitaxial orientation with respect to the substrate. The measured GISAXS intensity distributions are modeled by the Monte Carlo method taking into account the orientational distributions of NWs, a variety of their cross-sectional shapes and sizes, and roughness of their side facets. The cross-sectional size distributions of the NWs and the relative fractions of $(1\bar{1}00)$ and $(11\bar{2}0)$ side facets are determined.

cond-mat.mtrl-sci↗

arXiv2020

Small-angle X-ray scattering from GaN nanowires on Si(111): facet truncation rods, facet roughness, and Porod's law

Small-angle X-ray scattering from GaN nanowires grown on Si(111) is studied experimentally and modeled by means of Monte Carlo simulations. It is shown that the scattering intensity at large wave vectors does not follow Porod's law $I(q)\propto q^{-4}$. The intensity depends on the orientation of the side facets with respect to the incident X-ray beam. It is maximum when the scattering vector is directed along a facet normal, as a reminiscence of the surface truncation rod scattering. At large wave vectors $q$, the scattering intensity is found to be decreased by surface roughness. A root mean square roughness of 0.9~nm, which is the height of just 3--4 atomic steps per micron long facet, already gives rise to a strong intensity reduction.

cond-mat.mtrl-sci↗